This invention relates to concrete testing methods and apparatus and, more particularly, provides a method and apparatus for producing a test sample of concrete that is poured in place and later removed for testing.
In the construction of highways, buildings, and other structures utilizing concrete, it is necessary from time to time to test the strength of a sample of the poured concrete after it has cured to ensure that it has sufficient structural strength required for a particular installation. The most common method of testing concrete has been to produce a concrete test cylinder by pouring fresh concrete into a cylindrical mold, separate from the actual pour, consolidating it, and then leaving it to set. The following day the cylinders are picked up and delivered to a laboratory where the molds are removed and the cylinders are cured under laboratory conditions. After curing, the cylinders are tested for compressive strength at some specified time, most commonly the time at which a particular process is to be accomplished at the building site in which the concrete strength becomes a relevant factor. The use of separately poured and cured test cylinders is a long-used system and demonstrates the potential strength of concrete but is found to have certain drawbacks.
Laboratory test cylinders are largely ineffective in representing the in-place strength of concrete in that they are cured under circumstances that are entirely different from those to which the concrete poured in the field is subjected. The differences between the laboratory test cylinder and the actual concrete in the field have been demonstrated to be significantly more acute early in the setting cycle (less than two weeks) than they are late in the cycle (longer than one month). In today's fast-track construction process, it is critical to maintaining the schedule that early concrete strengths be accurately determined to allow for such activities as form and shoring removal, post-tensioning, panel tilting, and other activities. All of these activities need to be performed as early as the strength of the concrete will allow in order to facilitate the schedule but, if performed too early, pose severe safety hazards and jeopardize both the schedule and cost of the project by making failure of the structure possible when loads are applied or shoring is removed. Therefore, it is essential to be able to obtain accurate and timely information as to the strength of the actual concrete in place.
Concrete gains strength through an exothermic reaction between the cement and the mixing water. As in all such reactions, material temperatures are a major determining factor in the speed with which the reaction and corresponding strength gain occur. Because laboratory-cured test specimens are cured at temperatures often completely different from those of the work in the field, they often cannot represent the actual in-place strength. In an effort to solve some of the problems in laboratory-cured test samples, other systems have been tried to achieve a better representation of the in-place strength of the concrete. Presently, the most widely used alternate method for producing a test cylinder is the field-test cylinder. The field-test cylinder is essentially the same as the lab test cylinder in form but, rather than being cured in the laboratory, is left out in the field with the work until an activity requiring a given strength is scheduled. The field-test cylinder is then taken to the laboratory to be tested. The use of the field-test cylinder often produces test results that are more representative of the early in-place strengths of the concrete than are the lab-cured cylinders but there are still some serious drawbacks. The field-test cylinders are a completely different mass from the work in place. Testing professionals argue that this difference in mass allows the weather conditions, particularly if adverse, to have a much greater effect on the field-cured test cylinders than they do on the actual work in place.
Other methods for quick in-place testing of concrete utilize impact rebound hammers or probes shot from powder-actuated devices that actually impact the in-place concrete. Also, at times, rods are placed in the fresh concrete to be tension-removed later as an indicator of strength. The accuracy and simplicity of these methods have always been questioned and they have not gained widespread acceptance. One system for in-place testing that does work well is a system wherein a thermocouple is placed in the actual poured concrete to monitor the curing temperature. The thermocouple output is connected to a microprocessor controller that, in turn, controls a heater in a metal mold. A test sample of concrete is poured into the metal mold and the microprocessor, using the data gathered by the thermocouple in the actual workpiece, controls the temperature of the mold so that the concrete in the metal test mold is at the same temperature as the actual work in place as it cures. Using a heat-controlled test mold produces a sample that provides accurate data as to concrete strength but is prohibitively expensive for most general field use. Another method of obtaining a sample that can be accurately tested for strength is to drill a core sample from the actual work at the desired time, using a diamond-core drill. While this method produces an accurate sample, it also is quite expensive and takes a long time to complete. Additionally, core drilling can be dangerous if the core is drilled in the wrong location, cutting reinforcing steel or post-tensioning cables. Core drilling is also a messy procedure due to the water that is used to cool the drilling bit. Typically, at the present time, core drilling is used only in certain limited circumstances, such as to verify low strengths that have been reported by other testing means.
It is therefore an object of the present invention to provide a method and apparatus for producing a test sample of concrete that has been cured in conditions identical to those in which the field piece has been cured. It is a further object of the invention to provide such a sample by a method that is quick and easy to use and also relatively inexpensive when compared to present methods.